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Dissertation zur Erlangung des Doktorgrades der Fakultät für Chemie und Pharmazie der Ludwig-Maximilians-Universität München

Low Sensitivity Energetic Materials

Vorgelegt von Jan Matthew Welch

Aus

Albany, New York, Vereinigte Staaten von Amerika

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Erklärung

Diese Dissertation wurde im Sinne von § 13 Abs. 3 bzw. 4 der Promotionsordnung vom 29. Januar 1998 von Prof. Dr. Thomas M. Klapötke betreut.

Ehrenwörtliche Versicherung

Diese Dissertation wurde selbständig, ohne unerlaubte Hilfsmittel erarbeitet. München, den ………

……… (Jan M. Welch)

Dissertation eingereicht am:

1. Gutachter Prof. Dr. Thomas M. Klapötke 2. Gutacher Prof. Dr. Jürgen Evers

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Acknowledgement

As always (and as it should be) the first person a student should thank is the bill-payer (better known as a mentor), in this case, Prof. T. M. Klapötke, who for the past 4 years has done much more than just paying bills and being inordinately patient with a stubborn foreigner. All that really needs saying at this point is “thank you for the opportunity of a lifetime.” I’ve tried to seize it with as all the enthusiasm I could. Prof. J. Evers’ boundless enthusiasm, knowledge, humor and encouragement made a good deal of this work not only possible but fun to boot. Prof. K. Karaghiosoff managed to brighten even the dullest days with a never ending supply of stories and good humor. Also, worth mention are crystallographic aid and NMR assistance at all hours of the day or night on any day of the week. For helping me get off on the right foot both in the lab and out I owe a great deal to Dr. M-j Crawford. She is also the only person I know who has managed to share working space with me for nearly 4 years; that in and of itself speaks volumes. I’m also grateful for advice, both chemical and career from Dr. B. Krumm as well as encouragement when things got tough. Dr. Peter Mayer also appears here not only for measuring many of the structures in this work but also for having patience with an X-ray novice and supplying many of the files critical to this work more than once. Lastly, but certainly not least, I want to thank Ms. I. Scheckenbach, without whom I would be, among very many other things, homeless.

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1 General Introduction 1

1.1 Background and Terminology 1

1.2 Chemistry and Energetic Materials 4

1.3 Goals of this Study 9

1.3.1 Chapter 1: Energetic Salts 9

1.3.2 Chapter 2: Temperature dependent studies of FOX-7 10

1.4 References 10

2 Ionic Energetic Materials 11

2.1 Introduction to ionic energetic materials 11

2.2 Results and Discussion 14

2.2.1 Synthesis 14

2.2.2 Vibrational Spectroscopy 17

2.2.3 NMR Spectroscopy 23

2.2.4 X-ray Structures 27

2.2.5 Energetic Properties 64

2.3 Conclusions 75

2.4 Experimental 76

2.4.1 Caution 76

2.4.2 General 76

2.4.3 Bomb Calorimetry 77

2.4.4 X-ray Crystallography 77

2.4.5 Computational Methods 78

2.4.6 Syntheses 79

2.5 References 93

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3.1 Introduction 101

3.2 Results and Discussion 104

3.2.1 DSC 104

3.2.2 Temperature resolved qualitative analysis of bulk material 108

3.2.3 Single crystal X-ray structures 116

3.3 Conclusions 167

3.4 Future Work 169

3.5 Experimental Section 172

3.5.1 Caution 172

3.5.2 General 172

3.5.3 Single Crystal Experiments 173

3.5.4 Powder Diffraction Experiments 174

3.6 References 174

4 Appendices 177

4.1 Chapter 1 177

Brief description of the graph set description of hydrogen-bond patterns 177

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1 General Introduction

1.1 Background and Terminology.

Development of energetic materials, in other words, substances that burn, combust rapidly or explode under certain conditions constitutes one of mankind’s early scientific investigations. Although, explosive materials were developed later than other early human technologies (2nd century BC), combustion and the ability to control it is considered, one of the defining technological achievements of early humans. The history of energetic materials is well chronicled elsewhere and therefore mentioned here only in passing.1, 2 Early energetic materials were used primarily for entertainment (fireworks), but their potential in the realm of armed conflict was also recognized. An understanding and application of energetic materials in conflict has served, throughout history, as the key technological advantage of one civilization over another. In modern times, however, the manifold application of energetic materials reaches well beyond those original uses in entertainment and warfare. Today, energetic materials find use in mining, construction, demolition, safety equipment (signal flares, fire suppression systems, etc.), rocketry and space exploration among other civil and military applications.

Having established the utility and wide area of application of “energetic materials,” definition of the terms used above is helpful for the sake of clarity. As suggested above, “energetic materials” can be divided into several classes, namely, explosives, propellants and pyrotechnics, each of which requires a definition of its own. Explosives are most often vaguely defined as “a solid or liquid substance (or mixture of substances) which is in itself capable by chemical reaction of producing gas at such a temperature and pressure and at such a speed as to cause damage to the surroundings.” Propellants may be defined as substances or mixtures of substances that burn rapidly or deflagrate (deflagrate is defined specifically below) releasing a significant volume of gas at a rate sufficient to raise pressure and provide propulsive force (or impulse). Whereas the purpose of explosives and propellants is the

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transfer of stored chemical energy to macroscopic kinetic energy, the purpose of pyrotechnic substances and mixtures is to use this stored chemical energy to generate defined visual and acoustic effects.1, 2

The primary objectives of this work are in the area of explosives and to a lesser extent propellants. Therefore, further discussion of pyrotechnics is omitted. However, several more scientific definitions of terms concerning explosives and propellants may be useful. Firstly, in order to more precisely define the term “explosives” a definition of an “explosion” is necessary. Unfortunately, “explosion” is another term lacking a specific, scientific definition. An explosion is vaguely defined as occurring when a large amount of accumulated energy is released suddenly. The source of the energy released maybe physical, chemical or nuclear.1, 2 Therefore, rather than defining chemical explosive materials redundantly as those that, under certain conditions, release their energy suddenly, perhaps it is useful to discuss “explosions” in terms of the rate at which chemical energy is released.

Detonation is the fastest release of chemical energy and is propagated, not by heat transfer, but by a supersonic shockwave traveling though the materials. The shockwave propagation is maintained by the rapid release of chemical energy immediately behind the wave front. The shockwave exiting the material results in the destructive or propulsive force of the material. Deflagration reactions proceed through energetic materials at slightly subsonic linear velocities and are propagated principally by heat transfer. Deflagration can be considered an intermediate state between detonation and combustion. Combustion or simple burning reactions occur at slower rates and are defined as complex self-sustaining exothermic oxidation reactions. Therefore, in this work, chemical explosives are treated as compounds capable releasing their energy in the form of a detonation and these materials will be discussed principally in terms of their detonation properties. Propellants on the other hand, may be explosive materials, but release their energy principally through gas generation as the result of a deflagration.1, 2

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As evidenced by their definitions, explosives and propellants have different and varied applications. Explosive and propellant materials and mixtures can be further classified depending on ease of initiation to detonation or deflagration. The principal feature of primary explosives is facile initiation to detonation due to a very rapid intrinsic transition from burning to deflagration to detonation. Detonation of primary explosive materials is most often initiated by mechanical, electrostatic or thermal stimulus. Secondary explosives, on the other hand require a much larger stimulus to initiate detonation. Normally the stimulus initiating detonation of a secondary explosive is the detonation shockwave generated by a primary explosive. Although they are more difficult to detonate, secondary explosives generally have significantly higher performances (in other words release far more energy, more rapidly) than primary explosives. Within the secondary explosive class, two further types (classifications) of explosives (High Energy Density Materials, HEDMs and Low Performance Energetic Materials, LPEMs) have been heavily investigated in recent years. The purpose of HEDMs is to produce the highest performance possible, while the purpose of LPEMs is to sacrifice a small amount of performance to obtain the lowest sensitivity possible.1, 2

Propellants may also be divided into several classes and categories on the basis of application and composition including single, double and triple base solid propellants, composite propellants as well as liquid mono- and bipropellants. Single, double and triple base solid propellants contain one, two or three energetic components, respectively. The main ingredient used in solid propellant mixtures is nitrocellulose (NC). Double base propellants add nitroglycerin (NG) to the NC base and triple base propellants add both NG and nitroguanidine (NQ) to the NC base. Supplementing the energetic component(s) in classical solid propellants are a variety of substances including stabilizers, plasticizers, coolants, surface moderants and lubricants, flash inhibitors and decoppering and anti-wear agents. In addition to NC based propellants solid propellants, modern composite propellants

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based on a secondary explosive mixed with a plasticizer and binder are also in common use. Solid propellants are found in a variety of applications ranging from small and large caliber firearms to rocketry. In addition to solid propellants, liquid propellants are often used in rocketry and space exploration. As stated above liquid propellants may either be single component monopropellants such as hydrogen peroxide or anhydrous hydrazine or bipropellents which are mixtures of a liquid fuel and liquid oxidizer.1, 2

1.2 Chemistry and Energetic Materials.

Having defined the terminology specific to “explosions” and established the various classifications of energetic materials, the physical and chemical properties of energetic materials can easily be discussed. Sensitivity and performance are two of the most important physical properties of energetic materials and have already been mentioned above. Specifically, the sensitivity of an explosive substance may be defined as the ease with which detonation or deflagration is initiated by physical stimuli. These stimuli include friction (thermal and kinetic energy), impact (kinetic energy), electrostatic discharge (electrical energy) and thermal shock (thermal energy). Depending on the particular physical stimulus, the magnitude of the stress on the material required for ignition is more or less quantifiable. Performance of energetic materials may also quantified, although the physical quantities used depend on the type of reaction (detonation or deflagration), and are sometimes difficult to assess experimentally. Measures of detonation performance include detonation pressure (Pdet,

the pressure at front of the detonation shockwave) and velocity (D, rate of propagation of detonation shockwave), heat (Qdet, thermal energy released during detonation at constant

pressure) and temperature (Tdet, temperature of the detonation products) of detonation and the

volume of gas produced by the detonation (Vgas). Deflagration (propellant) performance is

most often gauged by specific impulse (Is, related to pressure and velocity of gas exiting a

rocket motor), the volume of gas produced by the deflagration (Vgas) and linear burning rate of

the material.

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Detonation and deflagration are largely defined by physical quantities which are ultimately related to the chemical properties of the material. The simplest chemical properties of the material, chemical composition (most often discussed in terms of percentage of nitrogen, N%, and oxygen balance (Ω)) and molecular structure (atomic connectivity), largely determine the density (ρ) and energy of formation (ΔfU) of the material. Since the rate at which the

detonation shockwave is limited by the density of the material through which it is propagating and is sustained by the energy released immediately behind the wave front, density and energy of formation (which are largely determined by chemical composition and molecular connectivity) underlie all of the above mentioned measures of energetic performance. Energy of formation also directly influences heat of detonation and the composition of the material determines the volume and type of gas released.

Just as performance is related to the chemical properties of energetic materials, so is sensitivity. The sensitivity of an explosive substance is a direct reflection of the ability of that substance to dissipate sudden energetic input, without undergoing decomposition (detonation, deflagration or combustion). The ability to dissipate energy without breaking chemical bonds (decomposition) is directly related to the density (lower density materials can dissipate energy more efficiently), molecular structure (certain functional groups are more susceptible to decomposition) and intermolecular interactions within the material (strong interactions between molecules lead to more efficient dissipation of energy).

As stated above modern energetic materials have a myriad of both military and civilian applications. Knowledge of the relationships between chemical and energetic properties can thus be used to develop new energetic materials tailored to specific applications. Modern, practical research and development of such materials has, in particular, three focuses. Classically as well as currently, the development of more powerful energetic materials, primarily for military application, is a principal aim of this field. Although an interest in safety and ease of handling has been more or less continuously present since the development

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of commercial explosive materials, in recent times the interest in developing low sensitivity and insensitive materials with long shelf-lives has become considerable. The most recent development in this field is that of “green” energetic materials, in other words, compounds that can be used or disposed of with the least possible environmental impact aside from that of the initial detonation.

Since this work focuses on secondary explosives and propellants, a detailed discussion of developments and properties of primary explosives will not be undertaken. Classically, secondary explosives have been based on molecules such as NG (nitroglycerin), TNT (2,4,6-trinitrotoluene) and RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) containing O-NO2, C-NO2

and N-NO2 functional groups, respectively. In the case of NG and TNT the energy of

detonation is obtained primarily from oxidation of carbon and the energies of formation of both compounds are negative. For RDX the energy of detonation is obtained by both the oxidation of carbon and the formation of molecular nitrogen. Due to the presence of the N-N bonds in the nitramine moieties, the energy of formation of RDX is positive and in addition to a high density, this results in a significantly higher performance than that of NG or TNT. The structures, properties and performance of these classical CHNO explosives are shown in Figure 1.1.3

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ONO2

ONO2

O2NO

CH3

O2N

NO2 NO2 N N N NO2 NO2

O2N

Systematic Name

(Abbreviation) propanetrioltrinitrate 1,2,3-(NG) 2,4,6-trinitrotoluene (TNT) 1,3,5-trinitro-1,3,5-triazinane (RDX)

Tm (ºC) 13 80 ~200

Tdec (ºC) 200 300 213

N (%)c 18.5 18.5 37.8

Ω (%)d 3.5 -73.9 -21.6

ρ (g/cm3)e 1.59 1.654 1.82

o m f

Δ H (kJ/mol) -370 -67 67

Impact (J) 0.2 15 7.4

Friction (N) >360 353 120

D (m/s) 7600 6900 8750

Figure 1.1. Properties of NG, TNT and RDX.

To improve on the performance of any of these materials, it is necessary to increase density, change the chemical composition or the molecular structure in order to increase the energy of formation. From a synthetic chemical stand point, new molecular structures with more oxygen balanced formulae, higher nitrogen contents and thus higher heats of formation are simpler to design and synthesize than compounds that will pack more efficiently leading to higher densities. Therefore most recently developed HEDMs use one or both of the following strategies to increase energy of formation: the use of strained cage structures as in molecules such as TEX (4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazaisowurtzitane), CL-20 (2,4,6,8,10,12-hexanitro-hexaazaisowurtzitane) and TNAZ (1,3,3-trinitroazetine) or the use of molecules containing large numbers of covalently bonded nitrogen atoms (nitrogen catenation) as in the primary explosive tetrazene (1(5-tetrazolyl)-4-guanyl tetrazene hydrate).

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N N N N N N NO2 NO2 NO2

O2N

O2N

O2N

O O O O N N NO2

O2N

N O2N NO2

NO2 N N NH H N N N H H N N NH2 NH2

TEX CL-20 TNAZ Tetrazene

H2O

Figure 1.2. Molecular structures of TEX, CL-20, TNAZ and tetrazene.

The concept of nitrogen catenation is also being utilized in the new area of “green” energetic materials. Since classical nitro CHNO explosives are not particularly environmentally benign, one of the major goals of green energetic materials research is the development of nitro-free energetic materials. Another major goal is the replacement of hydrazine containing liquid mono and bipropellants and of environmentally detrimental liquid (N2O4) and solid (NH4ClO4) oxidizers by more benign materials. The by-products of more

environmentally friendly, nitrogen-rich energetic materials should be mainly, if not exclusively, molecular nitrogen, carbon dioxide and water.4

Insensitive or low sensitivity secondary explosives are not a new concept, but are attracting increasing interest due to a need for stable, storable and transportable materials which are resistant to accidental ignition. The problem of accidental initiation has been present in the explosives industry from the time of Nobel’s first NG factories. Although advances in safety have been considerable, accidents still occur during production, transport and storage of energetic materials. The primary goal of research and development in the area of low sensitivity and insensitive energetic materials is reduce sensitivity to a minimum while sacrificing as little performance as possible. To this end the LPEM class of materials has developed. Although 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) was first synthesized nearly 120 years ago, it is currently the industry standard for a lower performance insensitive material.5 However, several new materials have been developed for low sensitivity

applications in recent years and research is still expanding.4

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NH2

O2N

NO2

NO2

NH2

H2N

NO2

H2N

H2N NO2

HN N NH O NO2 N+ N NH2

H2N

O -NO2

O2N

1,3,5-triamino-2,4,6-trinitrobenzene (TATB) 1,1-diamino-2,2-dinitroethene (FOX-7) 3-nitro-1,2,4-triazole-5-one (NTO) 2,6-diamino-3,5-dinitropyrazin-1-oxide (LLM-105)

Figure 1.3. Low sensitivity energetic materials

1.3 Goals of this Study.

The general area of this thesis is that of low- and insensitive secondary explosive materials and potential propellant compositions thereof. The development of new low sensitivity ionic materials incorporating the above outlined concepts of high nitrogen content, in order to improve performance as well as environmental compatibility (Chapter 1), is one of the two specific aims of this work. The second specific aim is the thorough structural investigation of FOX-7, a recently developed insensitive energetic material in the early stages of production, and its behavior as the physical extremes of detonation (high temperature and pressure) are approached (Chapter 2).

1.3.1 Chapter 1: Energetic Salts.

Two new families of energetic nitrogen-rich ionic energetic materials have been developed. The first family of salts utilizes combinations of cations based on 1-methyl-5-amino-2H-tetrazole (4) and 1,4-dimethyl-5-iminotetrazole (5) and simple oxygen- or nitrogen-rich

anions. The second family of ionic materials utilizes simple nitrogen-rich cations in combination with the anion of 5-nitro-2H-tetrazole. In addition, the cation of 5 was combined

with the anion of 5-nitro-2H-tetrazole generating a novel tetrazolium tetrazolate energetic salt. Each material studied, has been synthesized by scalable means from inexpensive readily

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available laboratory chemicals, fully characterized in bulk (by spectroscopic and analytical methods) and its molecular structure determined by X-ray crystallography. In addition to standard chemical characterization, the physical, thermochemical and energetic properties of each compound have also been thoroughly investigated and verified by computationally based methods when possible. The performance of each new substance has also been predicted using the EXPLO5 hydrodynamic code in order to identify compounds of particular interest.

1.3.2 Chapter 2: Temperature dependent studies of FOX-7.

The behavior of FOX-7 upon heating has been thoroughly investigated using differential scanning calorimetry (DSC), temperature resolved Raman spectroscopy, temperature resolved X-ray powder diffraction and temperature resolved single crystal X-ray diffraction. The occurrence of two solid-state phase transitions on heating has been verified and the structures of two high temperature polymorphs of FOX-7 (β- and γ-FOX-7) have been determined. On the basis of DSC measurements, diffraction data and structure determinations the mechanism and nature of the both transitions has been elucidated. Lastly, the physical changes caused by and motivating the phase transformations were examined.

1.4 References.

(1) Chimia, 2004, 58, 351-429.

(2) Akhavan, J., The chemistry of explosives. Royal Society of Chemistry: Cambridge, UK, 1998.

(3) Köhler, J.; Mayer, R., Explosivstoffe. Wiley-VCH: Weinheim, Germany, 1998. (4) Sikder, A. K.; Sikder, N., J. Haz. Mater., 2004, 112, 1-15.

(5) Pagoria, P. F.; Lee, G. S.; Mitchell, A. R.; Schmidt, R. D., Thermochim. Acta, 2002,

384, 187-204.

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2 Ionic Energetic Materials

2.1 Introduction to ionic energetic materials

As discussed in the general introduction, the primary goal of research concerning low sensitivity or insensitive energetic materials is to couple reasonable performance with decreased sensitivity. Unfortunately, as also mentioned in the general introduction, performance and sensitivity are linked both to each other and to the general physical and chemical properties of a substance. The primary chemical method of counteracting the correlation of high performance with high sensitivity is the synthesis of compounds which form extensive hydrogen bonding networks in the solid state such as FOX-71 or TATB2. Hydrogen bonded networks (especially between amino and nitro groups) stabilize, by several means, the material substantially. In addition to lower sensitivities, the performance of such materials is generally adequate since chemical composition is unaffected by strong hydrogen bonding (FOX-7, for example, shares a common empirical formula, CH2N2O2, with HMX and

RDX), density is generally increased and heat of formation only slightly decreased (less positive). Since the performance of an energetic material is most heavily dependent on density, increases in density tend to outweigh the adverse affect on heat of formation caused by strong hydrogen bonding. The density of such materials also benefits from the higher packing efficiency achieved through stronger intermolecular interactions.

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possible and chemical constitution and therefore density and energy of formation are thus easily adjusted by combining cations and anions with different compositions. In addition to ease of synthesis, ionic materials are defined by strong electrostatic interionic interactions in the solid state and often form strong hydrogen bonding networks, the result of which should be considerable stability as well as insensitivity. Furthermore, ionic energetic materials tend to exhibit lower vapor pressure (essentially eliminating the risk of exposure through inhalation) than their neutral non-ionic analogues. In short, ionic energetic materials should be cheaper, more flexible, safer and perform at least as well as their molecular counterparts.

3-5 With these properties in mind, two families of nitrogen-rich ionic materials were

synthesized. One family utilizes methylated 5-amino-1H-tetrazoles as cations with simple nitrogen- and oxygen-rich anions (ClO4-, NO3-, N3-, (NO2)2N-) and the other 5-nitrotetrazolate

in combination with simple, nitrogen-rich cations (ammonium, hydrazinium, guanidinium and amino- and polyaminoguanidinium).

The choice of methylated amino-1H-tetrazoles is motivated by recent studies of salts of 5-amino-1H-tetrazole 1 (5-AT)6 and 1,5-diamino-1H-tetrazole 2 (DAT)7, which showed

promising performance but suffer several shortcomings. Firstly, in the case of DAT salts, synthesis of the parent compound is difficult and hazardous.8 Secondly, in spite of their large number of electron pairs, 1 and 2 are both only weakly basic and therefore are only

protonated by strong acids, thus precluding their direct combination with slightly basic nitrogen and oxygen rich anions (e.g. dinitramide and azide). Therefore, in the case of 2, the

tetrazole ring was selectively methylated at the N4 position, generating the N5 protonated form of 3 with no strongly acidic protons and allowing for combination with a far greater

variety of anions. Since 1-methyl-5-amino-1H-tetrazole (4) is easily synthesized from

inexpensive reagents with no hazardous side products9, 10 and the hydroiodide of 5,

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energetic salts based on protonated and methylated 4, analogous to those of 2 and 3 which

showed promising properties, can be envisioned.

NH N N N NH2 N N N N NH2

NH2 N

N N N

NH

NH2 N

N N N NH2 N N N N NH

1 2 3 4 5

The second class of ionic energetic materials studied is based on the 5-nitrotetrazolate anion (NT-), which was first synthesized over 75 years ago by von Herz.12 Since its discovery, the metal and heavy metal salts of NT- have attracted much interest as primary explosives.12-16 More recently metal complexes (Zn and Fe) have been presented as potential “green” replacements for lead azide in initiators.17 In 1998 the free acid of the NT- anion (5-nitro-2H-tetrazole, HNT) was isolated and characterized for the first time along with a series of functionalized derivatives.18 Additional, neutral energetic derivatives have also appeared over the last few years.19 Finally, also in recent years, new energetic heterocycle based salts of HNT have appeared in the literature, many of which display promising ionic liquid properties.3, 5, 20-23

Considering the amount of literature concerning HNT and its derivatives and the recent interest in the area of ionic energetic materials, it is surprising that the simplest metal-free nitrogen-rich salts of HNT, remained largely uninvestigated. Further motivation for the investigation of simple, nitrogen-rich (ammonium, hydrazinium, guanidinium and amino- and polyaminoguanidinium) salts of HNT is provided by the promising properties of recently investigated simple nitrogen-rich salts of other tetrazole-based anions (e.g. 5,5’-azotetrazolate, ZT2-).24 Lastly, in light of the above mentioned interest in heterocyclic cation-NT- anion salts, the dimethylated cation of 5 was combined with the NT- anion and the

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2.2 Results and Discussion

2.2.1 Synthesis.

4 was prepared by methylation of the sodium salt of 5-aminotetrazole with methyl iodide or

dimethyl sulfate as described in literature.9, 10 The nitrate (6) and perchlorate (7) salts of 4

were prepared by protonation with the corresponding acid. Compound 8 was prepared by

regioselective methylation of 4 at N4 in acetonitrile with excess methyl iodide by a method

analogous to that used for the preparation of the iodide salt of 3.7 The salts of 5 were initially

prepared by exchange of iodide for nitrate (9), perchlorate (10), azide (11), dinitramide (12)

or 5-nitrotetrazolate (21) using the respective silver salt of the desired anion as shown in

Scheme 2.1. For the purposes of scale-up, alternative methods were utilized for the syntheses of 11, 12 and 21 due to the sensitivity of the silver salts of azide, dinitramide and

5-nitrotetrazolate. For 11 ion-exchange chromatography (as described by Christe, et al. in

reference 25) was utilized, for 12 the bis-pyridine silver dinitramide reagent described by

Franek et al.26 was used and for 21, 5 was reacted with ammonium 5-nitrotetrazolate

hemihydrate (13) under loss of ammonia. The reaction of 5 with free acid or ammonium salts

of the desired anion should yield salts of 5 in scaleable quantities with excellent yields

(Scheme 2.2). However, 5 was isolated by treatment of 8 with potassium hydroxide in

methanol and subsequent sublimation only after salts 9 - 12 had been thoroughly investigated.

Therefore such a method was utilized only for the large scale synthesis of 21.

+

MeOH

AgI

X =NO3-, ClO4-, N3-, N(NO2)2-, CN5O2

-N N N N

NH2+

I- AgX +

N N N N

NH2+

X

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15 +

THF

X =NO3-, ClO4-, N3-,N(NO2)2-, CN5O2

-N N N N

NH

HX or NH4X N

N N N

NH2+

X --NH3

Scheme 2.2. Synthesis of salts of 5 from 5 and free acids or ammonium salts.

Compound 13 used above, was prepared by a slightly modified literature method. Lee et al.

describe the material obtained as compound ammonium 5-nitrotetrazolate (14).27 In the

author’s experience, the preparation loosely described by Lee et al. and described in detail here yielded exclusively 13 as identified by elemental analysis and X-ray structure

determination. Lee et al. reported isolated yields of >70 %, whereas yields of ~50 % were obtained in this study. On scale-up (quadruple the procedure reported here) a yield of just over 60 % of 13 was obtained. In addition, Lee et al. also reported several properties of the

bulk material generated (density, heat of formation, sensitivity to impact, friction and electrostatic discharge as well as predicted energetic performance), which we found either to be in reasonable agreement with either values measured for 13 (sensitivities and density) or

values calculated based on the assumption that the material generated was 14 (heat of

formation, predicted energetic performance). Therefore, the author suggests that the material isolated by Lee et al. was 13 in slightly lower yield than reported.

Initially, compounds 13 - 19 (hydrazinium 5-nitrotetrazolate, 15, guanidinium

5-nitrotetrazolate, 16, aminoguanidinium 5-nitrotetrazolate, 17, diaminoguanidinium

5-nitrotetrazolate, 18 and triaminoguanidinium 5-nitrotetrazolate monohydrate, 19) were

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large scale (>1 g) manipulation of dry material is not advisable. Secondly, silver salts tend to be reduced by aminated guanidines and hydrazines to elemental silver. Therefore, even with short reaction times substantial decomposition of hydrazine and amino-, diamino- and triaminoguanidine moieties is observed.

N N

-N N

NO2

Ag+ + [Cat][X]

MeOH N

N

-N N

NO2

Cat+ + AgX

X = Br- or I

-Cat = NH4+, N2H5+, CH6N3+, CH7N4+, CH8N5+, CH9N6+ or CN5O2 -Scheme 2.3. Synthesis of NT salts by silver salt metathesis reactions

It is also foreseeable to use 5-nitrotetrazole itself as the transfer reagent. However the free-acid 5-nitrotetrazole is reported to be both significantly friction and impact sensitive as well as hygroscopic and is, therefore, not suitable for larger scale manipulation. Therefore, 13,

generated as discussed above, was used as a starting material. To generate compounds 15 –

17 free bases (hydrazine hydrate and guanidine) and hydrogen carbonates (aminoguanidinium

hydrogen carbonate, AGHCO3) were combined with 13 under loss of ammonia and in the

case of compound 17 carbon dioxide. In practice, compound 16 crystallizes preferentially

from a warm aqueous mixture of 13 and guanidinium nitrate (GNO3) and is thus most easily

generated by this method. Compounds 18 and 19 can be generated using barium

5-nitrotetrazolate and the corresponding aminated guanidinium hemisulfate. However, this method is also undesirable because barium nitrotetrazolate is a sensitive explosive material which forms with variable hydration. Diamino- and triaminoguanidinium hemisulfates are also not conveniently prepared on a laboratory or larger scale.28 Moreover, anhydrous triaminoguandinium 5-nitrotetrazolate (20) cannot be prepared directly by this method (19 is

dehydrated over P4O10 to form 20), since water is the only available, practical solvent in

which triaminoguanidinium hemsulfate dihydrate is soluble. Compound 18 was thus

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of ammonia, carbon dioxide water and sodium sulfate. A similar synthesis should be possible for 19 and possibly 20 directly from 1,2-diamino-3-ammonioguanidinium sulfate and 13, as

suggested in

Scheme 2.4, but has not yet been applied. Methods suitable for scale-up are shown in

Scheme 2.4 and all but those for the sensitive explosive materials 15 and 20 have been

applied on at least a ~5 g scale.

NO2 N

N N N

NH4+

• 0.5 H2O

EtOH / NH2NH2 • H2O

NO2

NH2NH3+

GNO3

H2O / -NH4NO3

NH2

H2N

NH2+

EtOH / AGHCO3

-NH3 / -CO2 / -H2O

NH2

HN NH2+

NH2

NH2

HN NH+

NH2

H2N

N H HN

NH+

NH2

H2N

NH2

EtOH

HDAGSO4 / Na2CO3

-NH3 / -CO2 / -H2O

-Na2SO4

NH2

HN NH+

NH3+ H2N

1-Amino-3-ammonioguanidinium sulfate (HDAGSO4)

CH3CN

HTAGSO4 / Na2CO3

-NH3 / -CO2 / -H2O

-Na2SO4

N H HN

NH+

NH3+

H2N

1,2-Diamino-3-ammonioguanidinium sulfate (HTAGSO4)

NH2

SO4

2-SO4

2-N N N N NO2 N N N N NO2 N N N N NO2 N N N N NO2 N N N N

Scheme 2.4. Applied and proposed scalable syntheses of simple, nitrogen-rich NT salts

from 13.

2.2.2 Vibrational Spectroscopy

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1,3-18

diaminoguanidinium and 1,2,3-triaminoguanidinium) and anions (nitrate, perchlorate, azide, dinitramide and 5-nitrotetrazolate).

The IR and Raman spectra of compounds 6 – 12 and 21 contain the following set of bands

which can be assigned to the cations: 3400 - 3100 cm-1 [ν(N-H)], 3000 - 2850 cm-1 [ν(C-H)], ~1680 cm-1 [ν(C1=N5) + δ(N5H2)], 1550 - 1350 cm-1 [ν(tetrazole ring), δas(CH3), δ(N4-H)],

~1380 cm-1 [δ(CH3)], 1350 - 700 cm-1 [ν(N1-C1-N4), ν(N-N), γ(CN), δ(tetrazole ring)], <700

cm-1 [δ out-of-plane bend (N-H), ω(N5H2)].29 The cations in salts 14 - 20 display well-known

sets of vibrations in the IR and Raman spectra. All of these salts show a set of strong absorptions in the IR and weak bands in the Raman spectra between 3500 - 2800 cm-1 corresponding to N–H stretching modes. N–H deformation vibrations are observed in their expected ranges as intense bands in the IR and weak bands in the Raman (1490 - 1390 cm-1 for 13 and 14 and 1700 - 1500 cm-1 for 15 - 20). In the case of 16 - 20, the N–H deformation

vibrations are strongly coupled to the C=N stretching modes, increasing both IR and Raman intensities. Additionally, in 15 - 20, an N–N stretching band is observed between 800 - 1000

cm-1 in both IR and Raman spectra.29

The nitrate anion, NO3-, shows a strong, broad IR absorption centered at ~1350 cm-1 and a

sharp band near 1050 cm-1 in the Raman. 30, 31 Compounds 6 and 9 show a broad IR signal

masked by cation absorptions with maxima at 1312 cm-1 and 1307 cm-1 respectively. In the Raman sharp, intense NO3- bands are observed in 6 and 9 at 1049 cm-1 and 1041 cm-1

respectively, as expected. Salts of the perchlorate anion, ClO4-, show a strong broad signal

with its maximum around 1090 cm-1 in the infrared spectra, and strong, sharp bands around 940 and 460 cm-1 in the Raman spectra.32, 33 As in the cases of

6 and 9, the perchlorate stretch

in the IR spectra of 7 and 10 is masked to a certain extent by tetrazolium cation absorptions

with two strong absorptions appearing in each spectrum at 1072 and 1033 cm-1 and 1108 and 1053 cm-1, respectively. Once again, Raman spectroscopy unambiguously confirms the

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19

and at 457 and 461 cm-1, respectively. The antisymmetric stretch of ionic azides is observed as a strong absorption at around 2000 cm-1 and the symmetric stretch as a weak absorption around 1300 cm-1 in the IR. In the Raman only the symmetric stretch is observed for ionic azides as a strong band around 1300 cm-1.25 Interestingly, in both the IR and Raman spectra of 11 both azide vibrations are observed (2018 and 1329 cm-1 in the IR and 2027 and 1331

cm-1 in the Raman). The presence of both bands in both IR and Raman spectra seems to indicate strong cation-anion interactions. The dinitramide anion N(NO2)2- has strong

absorptions at around 1530, 1445, 1345, 1183, and 1025 cm-1 in the infrared and strong bands in the Raman spectrum at 1335 and 830 cm-1.34 The IR and Raman spectra of 12 confirm the

presence of the dinitramide anion as evidenced by bands at 1529, 1446, 1183 and 1011 cm-1 in the IR and bands at 1329 and 825 cm-1 in the Raman.

The characteristic vibrations of the NT anion are either incompletely described in the literature in the case of the IR absorptions or wholly unknown in the case of the Raman bands. Therefore, in order to establish a characteristic set of vibrations for the NT anion, the vibrational frequencies and associated intensities and activities for the NT anion were calculated by quantum chemical methods, as described in the experimental section. In addition to establishing a characteristic set of bands in the vibrational spectra of compounds

13 - 21, quantum chemical calculations also facilitate the assignment of bands observed in the

experimental spectra to individual and coupled vibrational modes, which can in turn be used to derive structural information about the NT anion and its surroundings in the solid state.

The scaled, calculated IR and Raman frequencies calculated IR intensities and Raman activities are summarized in Table 2.1, along with average measured frequencies, qualitative IR intensities and averaged Raman activities (quantitative) from the vibrational spectra of 13 -

21. The experimentally observed vibrations were thus assigned by comparison to the

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20

Table 2.1. Calculated and measured IR and Raman frequencies, intensities (IR) and activities

(Raman) for the 5-nitrotetrazolate (NT) anion.

νcalc (cm-1)a νmeas (cm-1)b

(IR / Raman) IR intensity(Calc / Obs) Raman activity(Calc / Obs) Approximate Mode Assignment

1 1538 1533(6) / 1538(8) 74.1 / s 3.8 / 7.3 νasymm(NO2)

2 1396 1442(5) / 1443(3) 19.5 / s 0.18 / 6 νasymm(N1 – C5 – N4)

3 1383 1417(3) / 1418(4) 100 / vs 100 / 100 νsymm(NO2) + νsymm(N1 – C5 – N4),

“in phase”

4 1294 1316(3) / 1317(2) 12.6 / s 0.4 / 5.6 νsymm(NO2) + νsymm(N1 – C5 – N4),

“out of phase”

5 1182 1181(9) / 1180(3) 2.0 / m 2.6 / 3.7 νasymm(Tetrazole)

6 1163 1158(9) / 1164(7) 19.7 / m 4.0 / 4.6 νsymm(Tetrazole) + νsymm(NO2),

“in phase”

7 1024 1058(7) / 1059(7) 0.2 / w 8.0 / 50.9 δ(N1 – C5 – N4) + νsymm(NO2),

“in phase”

8 1018 1042(5) / 1044(5) 1.2 / vw 3.8 / 40.3 δasymm(Tetrazole)

9 966 1029(3) / 1030(6) 19.0 / w 64.4 / 7.6 ν(N2 – N3) + δsymm(Tetrazole),

“out of phase”

10 821 836(2) / 839(3) 14.4 / s 5.0 / 9.3 δ(NO2) + δ(N1 – C5 – N4),

“in of phase”

11 763 765(13) / 773(1) 2.4 / vw 0.8 / 3.3 γ(NO2) + γ(N1 – C5 – N4),

“out of phase”

12 718 725(13) / 730(11) 0 / vw 0.001 / 1.1 γ(Tetrazole) “in phase”

13 664 667(2) / 669(3) 3.3 / w 0.2 / 1 γ(Tetrazole) “out of phase”

14 522 n.o. / 540(4) 0.02 / n.o. 2.3 / 4.9 ω(NO2) + ω(Tetrazole),

“out of phase”

15 439 n.o. / 451(4) 0.2 / n.o. 1.8 / 2.9 ν(C5 – N5) + δ(NO2)

16 242 n.o. / 245(3) 0.2 / n.o. 0.3 / 3.4 ω(NO2) + ω(Tetrazole), “in phase”

17 218 n.o. / 208(8) 1.9 / n.o. 0.3 / 1.3 γ(N1 – C5 – N4)

18 57 n.o. / 62(5) 0 / n.o. 0.03 / 1 τ(C5 – N5)

a B3LYP / aug-cc-pVDZ Calculated frequencies; scaled by 0.9613.35 b Measured

frequencies; average values from IR and Raman spectra of compounds 13 - 21, with standard

deviations shown in parentheses; n.o. not observed. c Calculated intensities; percentages of the most intense signal; qualitative observed intensities; n.o. not observed. d Activities as percentage of the most intense signal; measured intensities are average values from 13 - 21. e

Approximate description of vibrational modes; ν, stretching, δ, in-plane bending, γ, out-of-plane bending, ω, in out-of-plane rocking, τ, torsion; asymm, asymmetric and symm symmetric; for atom numbering see X-ray section.

Remarkably, all 18 frequencies obtained from the computational results were observed in the vibrational spectra of compounds 13 - 21 with reasonably good agreement between the

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21

vibrational spectra of 13 - 21 occur at fairly consistent energies as indicated by the reasonably

narrow range of standard deviations observed for the average experimental frequencies (maximum 13 cm-1). As can been seen from the tabulated IR and Raman bands in the experimental section, some variation is observed in the observed vibrational frequencies for NT containing salts. Such variations are most likely due to differences in anion surroundings in the solid-state. In spite of these small differences, the IR and Raman data suggest that the observed geometry of the NT anion should be practically identical in all compounds in this study. This prediction is confirmed by X-ray crystallography (see Table 2.7).

In the case of compounds containing the HMAT+ and DMAT+ cations, IR spectroscopy can also provide detailed insight into cation structure and interionic interactions including hydrogen bonding, which in turn reflect the properties of the counter anions.36 A comparison of the IR spectrum of 4 to those of 6 and 7 shows substantial changes, shifts and broadening

of the bands observed in 4 when it is protonated at N4. The bands at 3328 and 3151 cm-1 in 4

are split and shifted to slightly higher wavenumbers in both 6 (bands at 3357, 3306, 3236 and

3159 cm-1) and 7 (bands at 3411, 3330, 3269, 3188 cm-1) indicating a strengthening

(shortening) of the N–H bonds in the exocyclic amino group. In the case of 6, two broad

signals appear at 2620 and 2360 cm-1 indicating the presence of strong hydrogen bonding involving the proton at the N4 position on the tetrazole ring. No such broad, lower energy N-H absorptions are observed for 7 indicating that only weaker hydrogen bonding should be

observed. Differences in N–H stretching band shifts observed in the IR spectra of 6 and 7

also seem to indicate the well known difference in basicity between nitrate and perchlorate. In 6 a smaller increase in amino N-H stretching frequencies is observed than in 7 because the

ring proton is more weakly bound in 6 and therefore places less electronic demand on the

amino group. In addition to this effect, stronger hydrogen bonding between the amino group protons and the nitrate anion seem to result in weaker amino N–H bonds in 6 and thus lower

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22

In addition to providing information about hydrogen bonding and N–H bond strengths, IR spectra also indicate other structural differences between neutral 4 and the HMAT+ cation.

The IR spectrum of 4 shows a strong band at 1667 cm-1 (with a shoulder at 1677 cm-1) and

another at 1596 cm-1 corresponding to amino group deformation and C=N ring stretch, respectively. 16 In spectra of salts 6 and 7 a single, higher energy band corresponding to the

coupled amino group deformation and exocyclic C=N stretch is observed at 1687 and 1684 cm-1 with a shoulder at 1702 cm-1, respectively and the C=N ring stretch band is no longer observed. These observations are explained perfectly by the structural changes expected on protonation of the tetrazole ring. The exocyclic C–NH2 bond length should decrease

(increase in stretching energy) and the endocyclic C-N bond length should increase (shift to lower wavenumbers). The observations of hydrogen bond strength and structural changes within the cation are verified by the crystal structures of these two materials (see X-ray discussion).

IR spectra of the salts of 5 also provide insight into the cation structure, hydrogen bonding

and the nature of the anions. Quaternization of 4 with methyl iodide yields 8. IR spectra

show a splitting of the asymmetric amino group stretching absorption observed at 3329 cm-1 for 4 into two bands (3249 and 3216 cm-1) for 8. The symmetric NH2 stretch remains a single

discrete band shifted to slightly lower energy (3082 cm-1, from 3155 cm-1 for

4) for 8. One

might expect quaternization of 4 to lead to higher energy NH2 stretching vibrations in 8.

However, this is not observed in the case of 8. Cation-anion hydrogen-bonding would seem

to explain the lower than expected NH2 stretching energies. Strong cation-anion

hydrogen-bonding is observed in the x-ray structure of 8. 10 An overview of the IR spectra of salts of 5

in this study shows that the energies of amino group stretching vibrations are almost entirely dependent on the counter anion. The highest energy NH2 stretching bands (3378, 3328, 3265

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23

stretching bands (3341, 3314, 3254 and 3139 cm-1) and thus the next weakest interionic interactions. 9 and 21 exhibit slightly stronger interionic interactions than those observed for

8 with broader, more intense amino group stretching vibrations appearing at 3276, 3223, 3166

and 3040 cm-1 and 3350, 3267, 3175 and 3031 cm-1, respectively. Lastly, the NH2 stretching

band in the IR spectrum of 11 is a, broad, intense signal with a maximum at 2874 cm-1

indicating the strongest cation-anion interactions found in for a salt of 5. The significant

strength of the interactions observed in 11 is also evidenced by the above mentioned

observation of the azide group antisymmetric stretch in the Raman spectrum, which, in purely ionic compounds, should not be observed. All of the observations of interionic hydrogen bond strength are confirmed by X-ray crystallography (see Table 2.6).

Further structural comparison of the HMAT+ and DMAT+ cations can be made by IR spectroscopy. The spectra of salts 6 and 7 show the above mentioned coupled amino group

deformation exocyclic C=N stretching vibration at 1687 and 1684 cm-1, respectively. The IR spectra of 8, 9, 10, 11, 12 and 21 all show the amino group deformation exocyclic C=N

stretching vibration as a single band at 1677, 1685, 1686, 1685, 1683 and 1690 cm-1, respectively, indicating that the ring and exocyclic amino group geometries in HMAT+ and DMAT+ should be very similar. Once again, X-ray crystallography confirms this suggestion (see Table 2.6).

2.2.3 NMR Spectroscopy

Both families of energetic salts and their precursors were fully characterized by multinuclear (1H, 13C, 14/15N and when possible 35Cl) NMR spectroscopy. The chemical shifts recorded for all nuclei as well as 1J, 2J and 3J (1H-15N) coupling constants, when measured, are reported in the experimental section. In the case of 4 all recorded signals and 1J(1H-15N)

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24

(see experimental), reported here for the first time, and those for compounds 6, 7 and 8 - 12

are in good agreement with typical values for 2J and 3J(1H-15N) coupling constants.38

In addition to providing a quick method to uniquely identify the compounds synthesized, protonation (6 and 7, PIS) and methylation (8 - 12, MIS) induced shifts (calculated as a

difference in 15N shift between analogous nitrogen signals in neutral 4 and salts of 4 and 5)

can be used to derive information about the structure in solution and hydrogen bonding (interionic interactions) of the salts of 4 and 5. In the literature, PIS and MIS values have

shown to be useful in unequivocally determining the site of protonation or methylation for a variety of heterocyclic compounds.7 So, for salts 6 and 7 as well as 8 - 12 the substantial

protonation and methylation induced shifts observed are tabulated in Table 2.2 (similar measurements of salts of 2 are shown for comparison).

Table 2.2. 15N and 13C Chemical Shifts (ppm) and Protonation (PIS) / Methylation (MIS)

Induced Shiftsa

N1 N2 N3 N4 N5 C5j C1/C4j

1b,h -137.1 -13.1 -13.1 -137.1 -338.9 157.2

1ac,h -165.2 (-28.1) -24.5 (-11.4) -24.5 (-11.4) -165.2 (-28.1) -329.1 (9.8) 152.4

2d,h -167 -5.5 -20.8 -97.5 -338.3 155

2aeih -164.9 (2.1) -21.9 (-16.4) -33.1 (-12.3) -170.4 (-72.9) -333.3 (5.0) 152.8 3af,h -167.9 (-0.9) -24.0 (-18.5) -35.3 (-14.5) -186.0 (-88.5) -319.0 (19.3) 148.1 33.6

4h -185 -23.2 2.2 -92.7 -338.2 156.4 32.0

6h -184.0 (1.0) -24.0 (-0.8) -13.5 (-15.7) -129.7 (-37.0) -331.4 (6.8) 153.6 32.9 7h -183.8 (1.2) -24.3 (-1.1) -17.4 (-19.6) -139.1 (-46.4) -329.5 (8.7) 155.2 31.6 8g,i -182.9 (2.1) -29.9 (-6.7) -29.9 (-32.1) -182.9 (-90.2) -318.0 (20.2) 149.1 34.7 9h -182.8 (2.2) -29.5 (-6.3) -29.5 (-31.7) -182.8 (-90.1) -320.4 (17.8) 148.9 34.2 10h -182.9 (2.1) -29.5 (-6.3) -29.5 (-31.7) -182.9 (-90.2) -320.0 (18.2) 148.5 34.0 11h -185.8 (-0.8) -30.7 (-7.5) -30.7 (-32.9) -185.8 (-93.1) -307.2 (31.0) 148.8 33.4 12h -182.7 (2.3) -29.4 (-6.2) -29.4 (-31.6) -182.7 (-90.0) -320.1 (18.1) 148.5 34.0 21i -182.6 (2.4) -29.6 (-6.4) -29.6 (-31.8) -182.6 (-89.9) -320.3 (17.9) 148.9 34.5

a PIS and MIS shifts in parentheses. b From Ref 5a. c 5-amino-1H-tetrazolium nitrate from

Ref 5a. d From Ref 5a. e 1,5-diamino-1H-tetrazolium nitrate from Ref 5a. f 1,5-diamino-4-methyl-1H-tetrazolium Iodide from Ref 5a. g 1,4-dimethyl-5-amimotetrazolium iodide from Ref 5c. h DMSO-d6. iCD3OD. j13C NMR shifts.

The PIS data for 6 and 7 show, as expected from the literature, the most significantly

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25

and thus indicate a strengthening of the C-NH2 and N-H bonds. Such an observation is in

excellent agreement with the solid state IR and Raman data as discussed above. The differences in cation structure due to differences in counter anion basicity observed in the vibrational spectra are also corroborated by the differences in PIS shifts for 6 and 7 at N4 and

N5. 6 shows less dramatic PIS shifts (greater structural similarity to 4) and thus stronger

interactions between HMAT+ (especially the acidic proton at N4) and nitrate than 7 for which

more dramatic PIS shifts (lesser structural similarity to 4) are observed indicating weaker

cation-anion interactions (hydrogen bonding) in 7.

The 15N NMR spectrum of the reaction mixture of 4 with methyl iodide shows only 3

resonances thus indicating selective methylation at N4 forming 8. In addition, the N1 (the N1

and N4 resonances are the same due to molecular symmetry) resonance observed is identical to that of N1 in 7 and shows an MIS of -90.2 relative to N4 of 4. The observed resonances

and MIS values observed and calculated for all salts of 5 except for 11 are all very nearly

equal within the tolerances of the measurement. The largest differences in resonances and MIS values are observed for N5, a fact which is explained by the NH2+ protons that interact to

varying degrees with the counter anions. 11 shows the strongest cation-anion interactions and

thus the largest positive MIS at N5 and slight but significant shifts throughout the entire spectrum. Such shifts for 11 fit nicely with vibrational data showing strong azide-NH2+

interactions in the solid state. For the remaining salts of 5 the MIS data are less conclusive

than vibrational methods, showing only similar amounts of interaction between the cations and anions.

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26

Interestingly, in the 14N NMR spectra of 13 - 20 signals corresponding to all three chemically

inequivalent nitrogen atoms in the NT anion are observed. The sharpest of the resonances, corresponding to the nitro group nitrogen, N5, is observed around -23 ppm with a line width between 60 and 110 Hz. The remaining two resonances are much broader (ν½ up to 990 Hz)

but no less characteristic and are observed at around 20 ppm (N2/3) and -60 ppm (N1/4). The observation and shifts of resonances for the tetrazole ring nitrogen atoms in the 14N spectra of

13 - 20 are consistent with those recorded for 5-trifluoromethyltetrazolate salts.39 The 14N

NMR spectrum of 13 is shown in Figure 2.1.

Figure 2.1. 14Nitrogen NMR spectrum of 13

2.2.4 X-ray Structures

Each compound synthesized was characterized by X-ray structure determination. Crystallographic data and structure determination details are presented in Table 2.3 (6 and 7),

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6 - 12 and 21 are shown in Table 2.6 and for 13 - 21 in Table 2.7. Hydrogen bond geometric

parameters are tabulated in Table 2.8 for 6 - 12 and 21 and Table 2.9, Table 2.11, Table 2.13,

Table 2.15, Table 2.17, Table 2.19, Table 2.20 and Table 2.22 for 13 - 20. Hydrogen bond

pattern graph-set matrices for 13 - 17, 19 and 20 are included in the text as Table 2.10, Table

2.12, Table 2.14, Table 2.16, Table 2.18, Table 2.21 and Table 2.23. 6 and 7 crystallize in the

same monoclinic space group P21/n but differences in the lengths of the crystallographic axes

and monoclinic angles indicate that the packing in the two structures is certainly different. Although both structures are composed of three dimensional hydrogen bonded networks consisting of seven crystallographically independent hydrogen bonds each, the packing around the planar nitrate anion in 6 is, for geometric reasons, different than the packing

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Table 2.3. Crystallographic Data and Structure Determination Details for 6 and 7.

6 7

formula C2H6N5+NO3- C2H6N5+ClO4

-formula weight (g/mol) 162.13 199.57 crystal system monoclinic monoclinic space group P21/n P21/n

a (Å) 10.6122(3) 5.2741(8)

b (Å) 5.3606(2) 20.696(3)

c (Å) 11.5508(4) 7.149(2)

α (º) 90 90

β (º) 97.663(2) 106.60(2)

γ (º) 90 90

V (Å3) 651.23(4) 747.8(2)

Z 4 4

ρcalcd (g/cm3) 1.653 1.773

μ (mm-1) 0.149 0.499

λ (Mo Kα, Å) 0.71073 0.71073

T (K) 200(2) 200(2)

reflections collected 8969 3813 independent reflections 1484 1465

Rint 0.0554 0.0372

observed reflections 1150 1333

F(000) 336 408

R1a 0.0355 0.0484

wR2b 0.1017 0.1061

weighting schemec 0.0483, 0.1647 0.0267, 1.2214

GOOF 1.076 1.161

number of parameters 125 133

a R

1 = Σ ||Fo| - |Fc|| / Σ |Fo|. b Rw = [Σ (Fo2 - Fc2) / Σ w (Fo)2]1/2. c w = [σc2 (Fo2) + (xP)2 +

yP]-1, P = (F

o2 - 2Fc2) / 3.

8 and 11 both crystallize in the orthorhombic space group Fddd and are closely related

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29

azide anions. In the case of the monatomic iodide anion in 8, the b-axis is slightly longer than

that of the azide anion in 11 since iodine has a larger van der Waals radius (2.15 Å) than

nitrogen (1.5 Å). 26 For the c-axis the situation is reversed with 11 having a longer axis than 8

since the linear azide anions in 11 are arranged parallel to the c-axis. Compound 10 (Figure

2.4) also forms dimeric pairs similar to those observed in 8 and 11, however due to the

tetrahedral shape of the perchlorate anion the compound crystallizes in the monoclinic space group P21/n rather than the orthorhombic Fddd, with substantially different lattice parameters

(Table 2.4). Lastly, structures 9, 12 and 21 crystallize in the non-centrosymmetric (chiral)

monoclinic space group P212121 with similar lattice parameters. As in the case of 8 and 11,

the packing patterns of these compounds are also related (Table 2.4). 9, 12 and 21 all form

infinite hydrogen bonded (3 H-bonds each, see Table 5), helical (source of chirality) chains of cations and anions. Interactions between the chains are once again limited weak van der Waals interactions. Although the dimensions of the crystallographic axes are similar for all three compounds, the axis of the helix is parallel to the a-axis in 9 and 21 and parallel to the

b-axis in 12, suggesting similarities between the preferred packing patterns of the NT anion

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30

Figure 2.2. View of a hydrogen-bonded dimer of 1,4-dimethyl-5-aminotetrazolium iodide (8)

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31

Figure 2.3. View of a hydrogen-bonded dimer of 1,4-dimethyl-5-aminotetrazolium azide

(11) with atom labels for one asymmetric unit and thermal ellipsoids shown for 50%

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32

Figure 2.4. Hydrogen-bonded dimer of 1,4-dimethyl-5-aminotetrazolium perchlorate (10)

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33

Table 2.4. Crystallographic Data and Structure Determination Details for 8 - 12 and 21.

8 9 10 11 12 21

formula C3H8N5+I- C3H8N5+NO3- C3H8N5+ClO4- C3H8N5+N3- C3H8N5+N3O4- C3H8N5+CN5O2

-mass (g/mol) 241.03 176.15 213.59 156.17 220.17 228.2

crystal system orthorhombi

c orthorhombic monoclinic orthorhombic orthorhombic orthorhombic

space group Fddd P212121 P21/n Fddd P212121 P212121

a (Å) 13.718(2) 5.3855(2) 5.7280(7) 12.351(2) 5.3985(6) 5.7089(10)

b (Å) 14.486(2) 10.8511(5) 10.9681(8) 13.952(2) 12.926(2) 12.8661(16)

c (Å) 16.281(2) 12.9152(6) 13.499(2) 17.180(2) 13.336(2) 13.3277(19)

α (º) 90 90 90 90 90 90

β (º) 90 90 92.24(2) 90 90 90

γ (º) 90 90 90 90 90 90

V (Å3) 3235.4(5) 754.75(6) 847.4(2) 2960.5(4) 930.6(2) 978.8(3)

Z 16 4 4 16 4 4

ρcalcd (g/cm3) 1.979 1.550 1.674 1.403 1.572 1.548

μ (mm-1) 3.891 0.135 0.446 0.106 0.14 0.127

λ (Mo Kα, Å) 0.71073 0.71073 0.71073 0.71073 0.71073 0.71073

T (K) 200(2) 100(2) 200(2) 200(2) 200(2) 200(2)

reflns collected 6527 4277 7096 6973 4786 6729

indep. reflns 964 1290 2001 730 1087 1650

Rint 0.036 0.0432 0.0438 0.062 0.062 0.0334

obsd. reflns. 786 886 1391 725 973 1523

F(000) 1824 368 440 1312 456 472

R1a 0.0169 0.0367 0.0333 0.0409 0.0488 0.0399

wR2b 0.0352 0.0903 0.0801 0.1069 0.1257 0.0990

weighting

schemec 0.0137, 3.5456 0.0503, 0.0000 0.0469, 0.0000 0.0579, 2.4663 0.0762, 0.0516 0.0660, 0.0323

GOOF 1.128 0.969 0.897 1.165 1.087 1.091

parameters 59 141 150 69 146 177

a R

1 = Σ ||Fo| - |Fc|| / Σ |Fo|. b Rw = [Σ (Fo2 - Fc2) / Σ w (Fo)2]1/2. c w = [σc2 (Fo2) + (xP)2 +

yP]-1, P = (Fo2 - 2Fc2) / 3.

Compounds 13 - 18 crystallize in a variety (P21/c, C2/c, P21/n and P21) of monoclinic space

groups whereas 20 crystallizes in the orthorhombic space group P212121 and 19 crystallizes in

triclinic P-1. From the unit cell parameters, no close relationships in packing may be deduced (no similar unit-cell parameters in structures with the same space groups). All salts of the NT anion studied here crystallize either in layered structures as in 14, 16 and 19 or, in the case of

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34

Table 2.5. Crystallographic data and structure determination details for compounds 13 - 20.

13 14 15 16 17 18 19 20

weight (g/mol) 141.11 132.1 147.12 174.15 189.17 204.18 237.22 219.20

crystal system Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Triclinic Orthorhombic space group C2/c P21/c P21/c C2/c P21/n P21 P-1 P212121

a (Å) 10.4860(13) 4.8436(2) 8.0124(3) 9.8240(3) 6.9447(5) 6.8162(9) 6.9805(9) 4.4024(4)

b (Å) 8.1461(10) 13.7981(6) 10.3237(5) 11.7601(4) 6.3716(6) 7.3468(10) 8.1249(9) 9.5261(7)

c (Å) 13.8381(17) 8.0853(4) 7.1531(3) 6.8736(2) 17.3196(12) 17.001(2) 9.1040(10) 21.6851(14)

α (º) 90 90 90 90 90 90 88.480(9) 90

β (º) 104.207(11) 97.2558(17) 106.186(3) 117.6043(13) 99.152(8) 92.774(11) 74.880(10) 90

γ (º) 90 90 90 90 90 90 81.314(10) 90

V (Å3) 1145.9(3) 536.03(4) 568.23(4) 703.72(4) 756.62(10) 850.4(2) 492.70(10) 909.42(12)

Z 4 4 4 4 4 4 2 4

ρcalcd (g/cm3) 1.636 1.637 1.72 1.644 1.661 1.595 1.599 1.601

μ (mm-1) 0.149 0.147 0.153 0.141 0.142 0.136 0.139 0.136 λ (Mo Kα, Å) 0.71069 0.71073 0.71073 0.71073 0.71073 0.71069 0.71069 0.71069

T (K) 140(2) 200(2) 200(2) 200(2) 200(2) 200(2) 140(2) 100(2) reflns collected 2875 6065 6560 5932 6344 6000 5044 11757 ind. reflections 1120 1229 1294 804 1492 2645 1926 1579

Rint 0.0292 0.1018 0.0806 0.0922 0.0493 0.0516 0.0366 0.0984

obs. reflections 1001 959 997 738 1721 1711 1749 921

F(000) 584 272 304 360 392 424 248 456

R1a 0.0329 0.0559 0.0576 0.0395 0.0499 0.0665 0.0406 0.0417

wR2b 0.088 0.1144 0.1124 0.1039 0.0872 0.1091 0.0963 0.0812

weighting

schemec 0.0499, 0.6428 0.0641, 0.0000 0.0594, 0.0801 0.0545, 0.2774 0.0544, 0.0000 0.0327, 0.0000 0.04010, 0.1780 0.0301, 0.0000

GOOF 1.05 1.057 1.047 1.08 1.131 1.046 1.093 0.99 no. of

parameters 107 99 112 69 146 301 189 172

a R

1 = Σ ||Fo| - |Fc|| / Σ |Fo|. b Rw = [Σ (Fo2 - Fc2) / Σ w (Fo)2]1/2. c w = [σc2 (Fo2) + (xP)2 +

yP]-1, P = (Fo2 - 2Fc2) / 3.

In the structures of 6 and 7 the HMAT+ cations are nearly identical within the limits of

structure determination accuracy as suggested by IR and Raman observations (Table 2.6). The HMAT+ cation is planar with exception of the methyl group protons which are staggered with respect to the ring in both 6 and 7 (Figure 2.5 and Figure 2.6). Unfortunately, we were

unable to determine the structure of 4 for comparison, however all bond lengths and angles in

6 and 7 are quite similar to those found in analogous salts of 1 and 2. The differences

observed are no greater than 0.01 A for analogous interatomic distances and no greater than 5 degrees for analogous angles.7 As in salts of 1 and 2, protonation is observed exclusively at

the N4 position, which is in excellent agreement with NMR observations and as mentioned above, the only slight variation in the HMAT+ cations observed is in the location of H4. Although the determination of absolute hydrogen atom position by X-ray methods is prone to large systematic errors stemming from the electronegativity of the atom to which the H atom is bonded,41 the HMAT+ cations in

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35

relative lengths of the N4-H4 bond should provide pertinent information about hydrogen bonding in these structures. Stronger hydrogen bonding is observed in 6 because the

calculated length of the N4-H4 bond (0.93(2) Å) is greater than that of the same bond (0.84(4) Å) in 7. As stated above, vibrational spectroscopy, as well as the acid-base properties of

nitrate and perchlorate suggest that exactly such a trend in hydrogen bond strength should be present (a more in depth discussion of hydrogen bonding in all salts of 4 and 5 follows

below).

Figure 2.5. Formula unit of 1-methyl-5-aminotetrazolium nitrate (6) with atom labels and

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36

Figure 2.6. Formula unit of 1-methyl-5-aminotetrazolium perchlorate (7) with atom labels

and thermal ellipsoids shown for 50% probability.

Table 2.6. Selected bond lengths (Å ) and angles (º) in compounds 6 - 12 and 21.

6 7 8 9 10 11 12 21

Bond Lengths

N1-C5 1.3386(19) 1.337(3) 1.330(3) 1.338(3) 1.328(2) 1.3373(15) 1.341(4) 1.339(2) N1-N2 1.3652(17) 1.370(3) 1.358(3) 1.362(2) 1.359(2) 1.3640(15) 1.356(4) 1.357(2) N1-C1 1.457(2) 1.463(4) 1.451(4) 1.450(3) 1.452(2) 1.4530(18) 1.466(4) 1.450(3) N2-N3 1.2739(18) 1.274(4) 1.277(5) 1.270(3) 1.274(2) 1.268(2) 1.270(4) 1.273(3) N3-N4 1.3604(18) 1.352(4) 1.358(3) 1.367(2) 1.361(2) 1.3640(15) 1.372(4) 1.361(2) N4-C5 1.3364(19) 1.337(4) 1.330(3) 1.337(3) 1.335(2) 1.3373(15) 1.345(4) 1.336(2) N5-C5 1.3173(19) 1.311(4) 1.311(4) 1.315(3) 1.314(3) 1.311(2) 1.304(4) 1.314(2) N4-Ra 0.93(2) 0.84(4) 1.451(4) 1.458(3) 1.456(3) 1.4530(18) 1.452(4) 1.456(3)

Bond Angles

C5-N1-N2 109.54(12) 109.6(2) 109.99(19) 109.32(17) 109.70(15) 109.31(11) 110.0(3) 109.51(16) C5-N1-C1 129.16(14) 129.6(3) 128.79(19) 129.64(19) 128.81(16) 128.80(12) 127.4(3) 128.16(16) N2-N1-C1 121.18(13) 120.7(3) 121.2(2) 121.02(19) 121.45(17) 121.65(11) 122.2(3) 122.30(15) N3-N2-N1 108.00(12) 107.7(2) 107.75(13) 108.52(17) 108.01(16) 108.22(7) 108.6(3) 108.15(15) N2-N3-N4 108.02(12) 108.1(2) 107.75(13) 107.77(17) 107.91(14) 108.22(7) 107.6(3) 108.15(16) C5-N4-N3 109.84(13) 110.3(2) 109.99(19) 109.58(18) 109.43(15) 109.31(11) 109.7(3) 109.39(15) N5-C5-N4 127.89(14) 128.2(3) 127.73(12) 127.7(2) 127.61(18) 127.54(8) 127.7(3) 127.43(16) N5-C5-N1 127.50(14) 127.6(3) 127.73(12) 127.5(2) 127.44(17) 127.54(8) 128.3(3) 127.78(18) N4-C5-N1 104.60(13) 104.3(2) 104.5(2) 104.80(19) 104.95(15) 104.92(15) 104.1(3) 104.79(16) C5-N4-Ra 128.79(19) 128.13(19) 128.90(17) 128.80(12) 128.9(3) 128.22(16)

N3-N4-Ra 121.2(2) 122.25(18) 121.60(15) 121.65(11) 121.3(3) 122.35(16)

a R = H (

6 and 7) and R = CH3 (8 - 12 and 21).

The structures of 8 - 12 and 21 also show nearly identical DMAT+ cations within the

Figure

Table 2.1.  Calculated and measured IR and Raman frequencies, intensities (IR) and activities
Figure 2.1.  14Nitrogen NMR spectrum of 13
Figure 2.2.  View of a hydrogen-bonded dimer of 1,4-dimethyl-5-aminotetrazolium iodide (8)
Figure 2.3.  View of a hydrogen-bonded dimer of 1,4-dimethyl-5-aminotetrazolium azide
+7

References

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